Rome:Total War is in all likelyhood THE most realistic strategy game out there today.
A typical battle:
2 units of Roman "Principes" (Heavy infantry), 1 unit of Equites (Cavalry) and 1 unit of archers are fighting a force of 4 Greek Armoured Hoplites (Spearmen).
The Greek phalanxes march forward confidently, knowing that no other infantry unit can match them. Just as they engage the Principes, the roman archers who have been lying hidden in a nearby wood emerge.The flanks of the phalanx are virtually defenseless, but they ignore the hail of arrows and push on, confident that once the principes are defeated, they will be able to deal with the archers. Suddenly, the Equites, who have circled around the phalanxes, attack their exposed rear. One unit breaks and routs, damaging the other phalanxes morale even further, until a second one routs, at which point the remaining 2 phalanxes flee the field as well.
Battles like these are ones you will have to fight regularly in Rome:Total War, but you may also fight battles with armies of 4000 men each, archers, cavalry, infantry, siege equipment, camels, and even elephants. The good balance of forces in Rome: Total War means that any force can be beaten, if you use the terrain, your units strengths and weaknesses and the morale of your men wisely.
The campaign mode, which is a turnbased map of the world in which you assemble and move your armies, is also well made, apart from the fact that the diplomacy system isn't very good (Treaty from Spain: Their demand: Please do not attack. Their offer: Accept or we will attack.)
All in all, Rome is definitely worth the money, as it is currently the most accurate and realistic RTS game.